The invention relates to demand cardiac pacers and particularly to a pacer in which the probability of inhibition of one or more heart stimulating impulses by interfering electrical signals is substantially reduced whether those interfering signals are physiological or non-physiological in origin.
The term "demand pacer" describes that particular type of pacer in which an electrical impulse generator is connected by wires and electrodes to the heart of the patient such as to deliver pulses of a heart stimulating mangitude at a rate approximately of a normal heart beating rate. In addition, the demand pacer includes an electrical amplifier connected also to the heart and having its parameters such that it is sensitive to the electrical manifestations of a heartbeat. Upon sensing a heartbeat by the amplifier further circuitry is activated to produce a triggering impulse which is delivered to the pacing pulse generator circuit to reset the timing cycle of the pulse generator and to inhibit the generation of the next otherwise delivered heart stimulating impulse. Such a pacer is described in U.S. Pat. No. 3,345,990, issued Oct. 10, 1967 to B. V. Berkovitz for "Heart-Beat Pacing Apparatus".
In a variation of the demand pacer the action of resetting the pacing pulse generator timing cycle upon detection of a natural heartbeat is accompanied by the delivery to the heart electrodes of a pulse of heartbeat stimulating magnitude which, being synchronous with a natural heartbeat, evokes no response. This variant is commonly described as a "ventricular synchronous demand pacer". Although offering physiological benefits similar to the first described type it has a disadvantage of wasting battery energy by supplying heart stimulating impulses when not needed.
In a further variation of the demand pacer, originated by applicant and incorporated in commercial designs since 1970, the action of resetting the pacing pulse generator upon detection of a natural heartbeat is accompanied by delivery to the heart electrodes of a pulse of less than heartbeat stimulating magnitude. This further variant allows the cardiologist to recognize normal pacer sensing actions in the presence of continuous natural heartbeats without waste of battery energy consequent to the ventricular synchronous demand pacer.
All these variations come within the definition of a "demand pacer", to which the present invention relates, although it must be pointed out that the response to electrical interfering signals is not normally the same in each of these variations. In the demand pacer in which sensed heartbeat produces resetting of the pacing pulse generator without coincident delivery of an impulse to the heart, the sensing of electrical interfering signals can cause complete suppression of the pacing function in the absence of natural heart rhythm -- a life threatening situation.
In the ventricular synchronous demand pacer, in which an impulse of heart stimulating magnitude is delivered to the heart electrodes upon sensing a natural heartbeat, the sensing of electrical interfering signals can cause a speed up of the rate of delivery of heart stimulating impulses and a consequent physiologically abnormal heart rate -- again, a life threatening situation. Thus, it can be seen that it is essential to minimize or eliminate the probability that any given interfering electrical signal might cause maloperation of a demand pacer.
To this end, prior art pacers have provided for tuning the demand sensing amplifier such as to make it most sensitive to those frequencies contained within that electrical manifestation of a naturally occurring depolarization of the cardiac muscle cells commonly described in clinical practice as the QRS sector of a cardiogram. Conversely, it is tuned to be less sensitive or insensitive to frequencies outside the QRS spectrum. The spectrum lies between approximately 20 Hz. and 100 Hz. with the frequency of peak energy density varying considerably from subject to subject.
With the tuning it is, of course, possible to eliminate the effect of interfering signals having frequencies outside the spectrum if of moderate magnitude. However, interfering signals can be of a magnitude many times greater than the heart's QRS signal and thus penetrate the tuning of the sensitive amplifier even when not containing frequencies within the amplifier's tuned spectrum.
Additionally, common sources of electrical interference produce frequencies within the amplifier's tuned spectrum. Included among these are sources of 50 Hz. to 60 Hz. power means, arc producing devices, such as welders or commutator machines, and other muscles of the body. Additionally, the interfering signals may not be a continuous wave of one discrete frequency but may contain many frequencies or be comprised of periodic or non-periodic short bursts of interference. As stated earlier, suppression of the heartbeat pacing impulses of a demand pacer by interfering signals is a life threatening event where the patient's heart is not beating in a natural rhythm at the time. Similarly, acceleration of the heartbeat pacing impulses is life threatening, irrespective of whether the patient has a natural heart rhythm at the time. In contrast, it is generally agreed that a situation in which the pacer delivers heart stimulating impulses at its preset rate even in the presence of naturally occuring heartbeats, can be tolerated by most patients for short periods.
There has been provided in the prior art designs such that the heart pacing impulses of the demand pacer are not suppressed or accelerated in rate (depending upon the form of the variation) but continue to be delivered at the preset rate even in the presence of certain forms of interference. Should the patient, at that time, have a natural heart rhythm, there will occur competition between the natural heart rhythm and the pacer stimulating rhythm. However, as previously stated, this is tolerable in a short term and unquestionably preferable to the life threatening alternative.
Examples of such prior art are two U.S. patents in the name of B. V. Berkovits, U.S. Pat. No. 3,528,428, issued Sept. 15, 1970 for "Demand Pacer" and U.S. Pat. No. 3,766,413, issued Oct. 15, 1973 for "Rate Discrimination Circuit". In these patents a demand pacer is described which, when sensing continuous wave electrical interference signals having a repetition rate much greater than a natural heartbeat repetition rate (for instance, 50 Hz. to 60 Hz. line interference) delivers heart stimulating impulses at its preset rate rather than having its heart stimulating impulses suppressed. In such a pacer the sensed signals are fed, after amplification, to a pulse forming circuit and then to a circuit having a resistive-capacitive (R-C) time constant.
When the time interval between two successive input events is similar to, or larger than, the R-C time constant, a large volage change occurs across the capacitor and it is this large voltage excursion which triggers or resets the pacer pulse generated timing circuit. Where the interval between two successive events is small compared to the R-C time constant, there is little change in the voltage across the capacitor at the occurrence of the second event and too little to trigger or reset the pacer timing circuit. This principle is most effective in preventing dangerous pacer operation in the presence of continuous wave electrical interference. However, the leading edge of the interference can trigger or reset the pacer pulse generator timing circuit once. Thus, if the interfering signal is in the form of short bursts, or is modulated or otherwise fluctuating in mangitude or frequency content, and the rate of the burst or modulation or fluctuation is not short compared to the R-C time constant, then the pacer output can be held suppressed for as long as the interference persists.
A further example of the prior art is a demand pacer which is refractory, that is, inoperative, for the first three-eighths of the pacer pulse generator timing cycle as measured from the last sensed event or delivered heart stimulating impulse. This device is set forth in a Cordis Corporation manual entitled Omni-Stanicor Implantable R-Wave Inhibited Cardiac Pacer in the Programmable Omnicor System, dated November 1972 and specifically pages 11-4 and 11-12 thereof. In this example, at the end of the refractory period is an interference sampling period having the duration of one-sixteenth of the pacer pulse generator timing cycle. During the interference sampling period any input events (voltage fluctuations crossing zero) are counted and, if exceeding a predetermined number (four), the input is classified as interference. The circuitry then, normally active to reset the pacer pulse generator timing cycle is deactivated for the remainder of the timing cycle and the heart stimulating impulse is delivered and the cycle recommenced. The effectiveness of this principle is limited in that interference can only be discriminated against by being detected during the short interference sampling period. Moreover, four or fewer pulses during the sampling period will not be recognized as interference while a pulse or burst outside the narrow sampling period will falsely reset the pulse generator. In other words, the principle is most effective in the presence of continuous wave interference, and only effective in the case of bursts of interference if the bursts coincide with the short interference sampling period.